Na⁺/H⁺ antiport activity in tonoplast vesicles from roots of the salt‐tolerant Plantago maritima and the salt‐sensitive Plantago media

Plantago species differ in their strategy towards salt stress, a major difference being the uptake and distribution of Na + ions. A salt‐sensitive ( Plantago media L.) and a salt‐tolerant ( P. maritima L.) species were compared with respect to Na + /H + antiport activities at the tonoplast. After exposure of the plants to 50 m M NaCl for 6 days isolated tonoplast vesicles of P. maritima showed Na + /H + antiport activity with saturation kinetics and a K m of 2.4 m M Na + , NaCl‐grown P. media and the control plants of both species showed no antiport activity. Selectivity of the antiport system for Na + was high and was determined by adding different chloride salts after formation of a Δ pH in the vesicles. Specific tonoplast ATPase activities were similar in the two species and did not alter after exposure to NaCl stress.